1. Field of the Invention
The present invention relates to a particle beam therapy system capable of high precision irradiation for treatment, and more particularly to a particle beam therapy system suitable for using a spot scanning irradiation method.
2. Description of the Related Art
In the recent aging society, a typical one of radiation therapies has attracted attention as one of cancer treatments since the radiation therapy is noninvasive to and has a low impact on human bodies. In addition, after the radiation therapy, the quality of life is highly maintained. Among the radiation therapies, a particle beam therapy system is a promising approach since the system provides an excellent dose concentration for an affected area of a patient. The particle beam therapy system uses a proton or a charged particle beam such as carbon, which is accelerated by an accelerator. The particle beam therapy system includes an accelerator, a beam transport system and an irradiation system (refer to Japanese Patent No. 2833602). The accelerator such as a synchrotron is adapted to accelerate a beam emitted by an ion source to a level close to the speed of light. The beam transport system is adapted to transport the beam extracted from the accelerator. The irradiation system is adapted to irradiate the beam on an affected area of a patient in conformity with the location and shape of the affected area.
Conventionally, in an irradiation system provided in a particle beam therapy system, a beam is formed by increasing the diameter of the beam by means of a scatterer and removing an outer periphery of the beam by means of a collimator in order to irradiate the beam on an affected area of a patient in conformity with the shape of the affected area. In this conventional method, the efficiency of using the beam is low, and an unnecessary neutron tends to be generated. In addition, there is a limitation in matching the shape of the beam with the shape of an affected area of a patient. Recently, the need of a scanning irradiation method has been increased as a higher precision irradiation method. In the scanning irradiation method, a beam having a small diameter is output from an accelerator, and bent by means of an electromagnet. An affected area of a patient is then scanned by means of the beam in conformity with on the shape of the affected area.
In the scanning irradiation method, a three-dimensional shape of an affected area is divided into a plurality of layers in a depth direction, and each of the layers is two-dimensionally divided into a plurality of portions. A plurality of irradiation spots is then set. Each of the layers is selectively irradiated with an irradiation beam by adjusting the energy of an irradiation beam dependent on the depth position of the layer. Each of the layers is two-dimensionally scanned with the irradiation beam by means of an electromagnet. The irradiation beam is irradiated on each irradiation spot with a predetermined dose. A method for continuously turning on an irradiation beam while the beam spot is moved from an irradiation spot to another irradiation spot is called raster scanning, whereas a method for turning off an irradiation beam while the beam spot is moved from an irradiation spot to another irradiation spot is called spot scanning. The spot scanning method is disclosed in Japanese Patent No. 3874766, for example.
In the conventional spot scanning method, a beam is irradiated on each irradiation spot with a predetermined dose under the condition that beam scanning is stopped. After the irradiation beam is turned off, the amount of an exciting current flowing in a scanning magnet is adjusted, and the beam spot is moved to the location of the next irradiation spot. To achieve high precision irradiation for treatment using the spot scanning method, it is necessary to position a spot of an irradiation beam with high accuracy and to turn on and off the irradiation beam at a high speed.
To obtain high accuracy of positioning of the irradiation beam spot, a known beam extraction method is used. In the beam extraction method, when a beam is extracted from a synchrotron, the size of the circulating beam is increased by means of a radio frequency (RF) power, and a particle having a large amplitude and exceeding a stability limit is extracted (refer to Japanese Patent No. 2596292). In this method, since an operation parameter of an extraction-related device for a synchrotron can be set to be constant during the extraction of the particle, orbit stability of the extracted beam is high. Therefore, an irradiation beam can be positioned with high accuracy, which is required for the spot scanning method.
However, it takes a certain time to block the extracted beam even when radio frequency (RF) power for extraction is turned off at the time of termination of irradiation on each spot. Thus, irradiation (delayed irradiation) occurs during the delay time. The delayed irradiation dose is not allowable in the spot scanning method in general. The beam extracted from the synchrotron is controlled to prevent the beam from reaching an irradiation system by turning on and off a shielding magnet provided in a beam transport system during movement of the beam spot from an irradiation spot to another irradiation spot. It is known that an irradiation beam is blocked by turning on and off a quadrupole magnet provided in a synchrotron at a high speed during movement of a beam spot from an irradiation spot to another irradiation spot (refer to JP-A-2005-332794).
Non-patent Document 1, “Fast beam cut-off method in RF-knockout extraction for spot-scanning”, Nuclear Instruments and Methods in Physics Research A489 (2002) 59-67 discloses an operation for turning off a radio frequency acceleration voltage to be applied to the synchrotron in synchronization with the turning-off of the extraction RF power as a method for reducing a delayed irradiation dose of a beam irradiated immediately after the termination of the irradiation on each spot in the case where the beam extraction method described in Japanese Patent No. 2596292 is used. In this method, the delayed irradiation dose of the beam irradiated immediately after the turning-off can be reduced. However, after the turning-off, the extracted beam with a small irradiation dose may leak in an area present between irradiation spots. Especially, the extracted beam with a relatively large irradiation dose may leak in an area present between irradiation spots during the change in the amount of the exciting current flowing in the scanning magnet in a remote spot irradiation method in which the irradiation spots are remotely located and are to be irradiated. In addition, the radio frequency acceleration voltage is turned off for a long time during movement of a beam spot from an irradiation spot to another irradiation spot in the remote spot irradiation method. The beam circulating in the synchrotron therefore becomes unstable. This results in a reduction in the quality of the irradiation beam.